On the steamship the steam is led not to the open air but to a vessel called a condenser the walls of which are kept cool by a continual circulation of cold water. The steam on entering the condenser at once collapses into water, leaving a vacuum.
A pump called the "air-pump" removes the water (which was once steam) from the condenser and also any air which might get in, with the result that the engine is always discharging its steam into a vacuum. Thus to the pressure of the steam is added the suction of the vacuum.
In turbine ships the cooling water for the condensers is circulated by powerful centrifugal pumps driven by subsidiary engines.
The steam is obtained from boilers of that special variety known as "water-tube."
The boilers with which most people are familiar are either Lancashire or Cornish, both sorts being large steel cylinders with two steel flues in the former and one in the latter running from back to front. The fire is made in the front part of the flue and the hot gases from it pass to the back and then along the sides and underneath through flues formed in the brickwork in which the boiler is set. Locomotive boilers, however, have no flues, but the hot gases from the fire in the fire-box pass through tubes which run from end to end through the cylindrical shell, each tube starting from the fire-box behind and terminating in the smoke-box in front. Thus we have tubes with fire inside and water outside: hence such boilers are called "fire-tube" boilers.
On many ships of the merchant type cylindrical boilers are used which combine the features, to some extent, of the Cornish and the fire-tube, since there is a flue running from front to back in which the fire is made and the hot gases return from back to front through a number of tubes which occupy the space
above the fire. Arrived at the front the gases pass upwards to the chimney.
Water-tube boilers are different from all of these, since in them the water is inside the tubes while the fires play around the outside. This enables steam to be got up very quickly, a matter of much importance for a warship which may be called upon to undertake some operation at a moment's notice.
The boilers are fed with water from the condensers, so that the same water is used over and over again. When coal is burnt it is put on the fires by hand, for although mechanical stoking is a great success on land, there are special difficulties which prevent its use at sea. It is becoming more and more the fashion now to burn oil instead of coal in several types of ships and in those cases the oil is blown in the form of spray into the furnace. This has many advantages, some of which are exemplified on a small scale by the difference between using a coal fire and a gas stove. Like the latter, the oil spray can be quickly lit when needed and as quickly extinguished. It can be regulated and adjusted with equal facility. Oil can be taken on board too through a pipe, silently and quickly and without the terrible dirt and the exhausting labour involved in coaling a big ship. Oil, too, can be taken on board at sea, from a tank steamer, almost as easily as it can be taken in ashore, whereas the difficulty of coaling at sea despite many ingenious efforts has never been solved quite satisfactorily. Finally, oil can be stowed anywhere, for the stokers do not need to dig it out with a shovel. Therefore it can be carried in those spaces
between the inner and outer bottoms which have to be there in order to give strength to the ship's hull but which would be quite useless for carrying coal. The advantages of oil fuel, therefore, are many and no doubt it will be used more and more as time goes on.